Large-scale pork production is a profitable industry in several regions of the Nation. Unfortunately, this economic success is often at the expense of local ambient air quality. The emission of hazardous and odorous gases from hog manure basins has polarized many rural communities. Public and regulatory pressure applied on the pork industry to control its air emissions already has slowed the expansion of the industry in certain regions. This has potentially severe economic consequences on rural and state economies. The continued economic viability of the regional pork industry and associated rural communities requires that pork production facilities adapt air pollution control technologies that ensure sustainable development. The same need for air pollution control exists in other industries with high-strength organic waste streams, such as the sugar refining, dairy, cannery, and food processing industries.
Modern, large-scale pork production is seldom cited as an example of sustainable development, because of the external and often unpaid costs associated with potential adverse effects on local water and air resources. The present trend is that the large-scale pork production facilities provide an increasingly greater fraction of the nation""s pork production. The spatial concentration of hogs and hog manure in such facilities results in the release of hazardous and odorous gases that exceed the natural ability of the atmosphere to maintain by dispersion concentrations below acceptable levels. Odors emitted from outdoor anaerobic basins can be detected by substantial portions of the population for considerable distances downwind of the basin. The lack of cost-effective technologies to control the emission of hazardous and odorous gases limits the growth of hog industry at a time when the demand for pork products is increasing. These constraints may result from tougher zoning regulations or fear of litigation. For example, the Sep. 12, 1994 issue of Farmweek magazine published a story describing how neighbors of an 800-hog operation in Iowa were awarded $45,000 in damages because of noxious odors, and another story describing how public concerns over hog odors in North Carolina forced county zoning boards to prohibit hog operations in certain locations (e.g. within one mile of towns). Thus, the spatial concentration of hogs and hog manure into larger operations results in a potential reduction in local air quality, which has both public health and public nuisance components.
With the exception of North Carolina, currently most of the pork production in the United States occurs in the corn belt states and parts of their immediate neighbors. Table 1 indicates the monetary value of pork production to the central United States. The crop farmers of this region depend directly and indirectly on the pork and other livestock industries. Midwestern farmers have invested in pork production facilities on their own properties or as part of cooperatives to provide income when crop prices are low. However, the present inability to control the emission of hazardous and odorous gases threatens the livelihood of region""s pork industry and the economic viability of rural communities. Such a loss of pork production would represent a significant loss to the Upper Midwest economy in terms of those activities involved directly (Table 1) and indirectly (corn and soybean growers, meat packers, distribution) with pork production.
Aside from aerobic biological treatment, few (if any) technologies have been proven effective in reducing the emission of hazardous air pollutants and odorous compounds from hog manure facilities under diverse environmental and managerial conditions. Part of the problem in developing manure additives for odor control is that the specific compounds responsible for mal-odors have not been satisfactorily identified. Without knowing what the chemical targets are for odor control, the end result is an essentially blind development of odor control products that are only partially effective (at best) in controlling odors.
The anaerobic biodegradation of hog manure generates several general classes of compounds that are considered hazardous, odorous, or both. These classes compounds include ammonia, reduced sulfur compounds (hydrogen sulfide, mercaptans), volatile fatty acids (n-butyric acid, valeric acid), phenolic compounds (phenol, para-cresol), indoles (skatole, indole), and volatile amines (putrescine, cadaverine). In general, the more offensive odorous compounds are associated with the anaerobic biodegradation of proteins. Because of the wide range in chemical properties represented by these classes of hazardous and odorous compounds, no one additive or process can be expected to control all offensive odorsxe2x80x94that is except for aeration. Aeration and the associated aerobic biological treatment can result in the biodegradation of all the above hazardous and odorous gases.
High operating costs are currently associated with aerating large volumes of high strength wastes. It has been suggested that aerobic biological treatment should only be used in situations where odor control is essential, because of the high operating costs. The cost for aerobic biological treatment of the entire manure stream generated by a 150 lb finishing pig is estimated to be about $7.00 per marketed pig. This high operating cost for complete aerobic treatment of hog manure continues to limit its acceptance by the pork production industry.
One means of reducing the costs of aeration is to reduce the volume of water that is aerated by operating outdoor hog manure basins as facultative lagoons. Facultative lagoons have been used in the municipal and industrial waste water treatment industries for decades as a means of meeting treatment objectives with reduced aeration costs. A facultative lagoon is one that has an aerobic (oxygen present) layer above an anaerobic (no oxygen present) layer. The offensive hazardous and odorous gases generated in the anaerobic layer are oxidized by the aerobic (oxygen-requiring) bacteria found in the upper layer of a facultative lagoon and converted into inoffensive products. However, as shown in Table 2, the operating costs associated with using the traditional waste water treatment approach to a facultative lagoon are still too expensive for odor control at most pork production operations.
Ideally, the operation of the facultative lagoon designed primarily for odor control should supply just enough aeration to biodegrade the hazardous and odorous gases that would otherwise be emitted into the atmosphere. Such an approach is feasible, because many of the offensive organic and inorganic gases released during the anaerobic decomposition of hog manure are preferentially biodegraded under aerobic conditions compared to the other components that make up the bulk biological oxygen demand of hog manure. Because the ideal facultative lagoon does not need to maintain sufficient dissolved oxygen concentrations to encourage nitrification, the oxygen demand associated with the biological oxidation of ammonia to nitrate can be removed from the oxygen requirement. Without nitrification, the aeration cost of odor control with the ideal facultative lagoon is $0.25/hog marketed.
As shown in Table 2, a facultative lagoon designed for odor control is significantly more cost effective than the other aerobic biological processes and has a theoretical minimum aeration cost of $0.25/hog marketed. This minimum ideal cost can be approached by aeration equipment that maintains redox potential of at least xe2x88x92100 mV in an aerobic mixing zone that is no greater than one foot thick.
Basin covers are another technology for reducing the emisssion of odorous gases from anaerobic lagoons. Covering at least part of an anaerobic lagoon""s liquid surface reduces odorous gas emissions. Non-porous and porous materials are used to cover anaerobic basins. Non-porous covers provide a means for containing and collecting the odorous gases released by the anaerobic lagoon. The air space maintained beneath a non-porous cover can be vented to air polution control equipment (e.g., flares, biofilters) or methane combustion equipment, which destroy the odors prior to release into the atmosphere. The stagnant air pockets within a porous cover provide additional mass transfer resistance for the movement of gases from the liquid surface of the anaerobic lagoon to the atmosphere. The slow, metered release of odorous gases allows air quality standards to be achieved without requiring gas collection and the installation of additional air pollution control equipment.
Both non-porous and porous covers also reduce hazardous and odorous gas emissions by maintaining quiescent conditions at the liquid surface of an anaerobic lagoon, and by allowing the attachment and accumulation of anaerobic bacteria on their bottom surfaces. The emission of hydrogen sulfide, ammonia, and other volatile gases are accelerated by water turbulence at the air/water interface. By preventing wind-induced mixing at the liquid surface of an anaerobic lagoon, covers reduce the emission of volatile gases by reducing water turbulence. Where a cover is in physical contact with the high-strength wastewater stored in an anaerobic lagoon, anaerobic bacteria will attach and colonize the underside of the cover. The resulting anaerobic microbial slime layer results in enhanced biological activity where it is most needed for odor controlxe2x80x94at the air/water interface. The beneficial effects due to the accumulation of anaerobic bacteria on the underside of the cover are typically associated with porous covers, because porous covers are almost always in contact with the liquid surface. The gas pockets that develop under non-porous covers and the air space that is maintained for venting any captured gas can limit the physical contact of the cover with the liquid surface.
The present invention relates to an odor control apparatus for a facultative lagoon. The odor control apparatus has two major components: aeration equipment and a basin cover. The aeration equipment provides sufficient dissolved oxygen under the basin cover to convert the otherwise anaerobic wastewater basin into a facultative lagoon. A facultative lagoon has an aerobic layer of water existing above an anaerobic layer of water. The aerobic layer has sufficient oxygen (e.g., redox potentials greater than xe2x88x92100 mV) to sustain a population of aerobic bacteria capable of biodegrading the odorous compounds generated in the anaerobic portion of the lagoon. The basin cover at least partially covers the lagoon and has three main functions. First, the cover mitigates the release of hazardous and odorous gases that would otherwise escape biodegradation in the aerobic layer. This would include the gas xe2x80x9cburpsxe2x80x9d and bubbles released from the anaerobic sediments located at the lagoon""s bottom. Second, the cover prevents the wind from vertically mixing the basin""s water column and destroying the layered structure of the facultative lagoon. Third, aerobic bacteria will colonize the underside of the basin cover that is in contact with the wastewater. The resulting retention of aerobic bacteria in the aerobic layer of the facultative lagoon increases the effectiveness and operational stability of the odor control apparatus. A cover also promotes the thermal stratification of the lagoon""s water column, which helps maintain separate aerobic and anaerobic layers in the lagoon.
Several different types of aeration equipment can be used in the present invention. A fundamental requirement for the aeration equipment is that sufficient oxygen be provided for biological odor control in the covered facultative lagoon without vertically mixing the lagoon""s water column.
An airlift aerator meets the aeration and mixing requirements for the present invention. An airlift aerator according to the invention includes a float or cables to sustain the device with respect to the lagoon surface. The aerator can include a U-shaped tube including a down flow leg and an airlift leg. The upper ends of the legs are upright such that the lower ends extend downward into the lagoon. The upper end of each leg is open to the lagoon near the surface thereof. Preferably the upper end of each leg has a horizontal portion forming an inlet in the case of the down flow leg, and an outlet in the case of the airlift leg. Preferably both the inlet and outlet are located in the targeted aerobic zone or layer. The targeted aerobic zone includes those portions of the lagoon that are located beneath the basin cover. A source of pressurized air is connected to the airlift leg at a location toward the lower end thereof. The air source is connected to a bubble diffuser open to the airlift leg.
The airlift aerator is operated by supplying air under pressure to the bubble diffuser. Bubbles are generated inside of the airlift leg and move upward. This creates fluid circulation in the airlift aerator. Oxygen deprived surface water is drawn into the down flow leg. This water travels down the down flow leg, around the connecting elbow, and up the airlift leg. Oxygen is transferred from entrained air bubbles to the oxygen deprived water. The aerated water is discharged to the lagoon surface layer creating and maintaining an aerobic layer to the lagoon approaching the ideal condition where just enough aeration is supplied to biodegrade the hazardous and odorous gases that would otherwise be emitted into the atmosphere.
Other types of aeration equipment can be used in the present invention, including mechanical aerators (e.g., aeration device described in U.S. Pat. No. 4,280,911), aspirators (e.g., aeration device described in U.S. Pat. No. 5,314,619), venturi injectors, and membrane-based aerators (e.g., aeration devices described in U.S. Pat. No. 5,034,164 and in U.S. Pat. No. 5,674,433). These examples of suitable aeration equipment would supply a jet of aerated wastewater into the aerobic water layer located beneath the basin cover. However, suitable aeration equipment need not be limited to equipment that generates a water jet. For example, a network of oxygen-permeable tubing (e.g., silicone tubing) or oxygen-permeable hollow-fiber membranes (e.g., microporous polyethylene hollow-fibre membranes could be attached or otherwise positioned beneath the basin cover. Air or pure oxygen would be supplied under pressure to the lumen of the tubing or hollow-fiber membranes. Aerobic bacteria would colonize the outer surfaces of the tubing or hollow-fiber membranes resulting in the biodegradation of odorous compounds.
In addition to dissolving oxygen in the aerobic water layer beneath the cover, the aeration component of the odor control apparatus could be used to dissolve other gases that are beneficial in the biodegradation of the odorous gases. For example, the dissolution of ozone in combination with aeration may improve odor control by chemically oxidizing certain odorous gases. Also, the oxidative cleavage of the aromatic rings found in phenols and odorous heterocylic compounds by ozone increases the susceptibility of the compounds to aerobic biodegradation.
The basin cover component of the present invention floats on the water surface and can either be made of non-porous (impermeable) or porous (permeable) materials. Non-porous covers can be made from plastic or hydrocarbon-based materials. A floating non-porous cover captures the air not dissolved by the aeration equipment and the odorous gases not biodegraded in the aerobic layer of the facultative lagoon. The collected gas needs to be vented and perhaps treated prior to emission into the atmosphere. Without venting, the non-porous cover would fill with air and no longer be in contact with the liquid surface. Cover inflation is undesirable because the loss of the cover/wastewater interface means the loss of the aerobic bacterial film on the underside of the cover from the facultative lagoon.
Porous floating basin covers can be made from a wide range of natural and man-made materials. Examples of natural porous covers include the fibrous crusts on dairy manure basins, straw, rice hulls, and cornstalks. Examples of man-made porous covers include plastic foam pellets, mats, tarpaulins, clay balls, and geotextile membranes (polypropylene felt). If the porous basin covers lack sufficient gas permeability to allow passage of the undissolved air from the aeration equipment, then vents will be required to exhaust the undissolved air and to prevent cover inflation. In addition to the aerobic bacteria on the underside of the floating cover, aerobic bacteria living within the humid, air-filled pores of a porous floating cover can also biodegrade odorous gases before emission into the atmosphere.
In terms of a method, a facultative lagoon is created and maintained by providing aeration device operating in an upper layer of the lagoon in combination with a cover covering part of, or substantially all of, the lagoon surface.